1. Field of the Invention
The present invention relates to field emission electron sources which emit electrons under applied electric field.
2. Description of Related Art
In the conventional art, field emission electron sources, which emit electrons under applied electric field, have been known and used in devices such as scanning electron microscopes, the electron beam lithography, and X-ray diffractometers. The emitted electron current (emission current) J from a cold cathode of the above-mentioned field emission electron source is expressed by the following equation (1):J=A·(F2/φ)exp(−BΦ3/2/F)  (1)where F is the electric field strength, φ is the work function, and A, B are constants.
According to the equation (1), it is clear that a larger emission current J can be obtained by having a smaller work function φ.
As a field emission electron source, a cold cathode made of tungsten covered by a thin metal film has been known (See, e.g., Patent Document 1 below). With the field emission electron source, a large emission current J can be obtained by having a tungsten single crystal covered with thin film coatings of pure gold and pure aluminum on the side plane of the (310) plane, and with a thin film coating of a tungsten-gold-aluminum ternary alloy at the pointed end of said (310) plane, which provide a lower work function φ than the tungsten work function (φ=5.5 eV).
In order to obtain an even larger emission current J, one may consider coating the cold cathode with a material whose work function φ is lower than that of the tungsten, such as the barium oxide and carbon etc. The work function of barium oxide is 2.0 eV and the work function of carbon is 4.5 eV.
A carbon nanotube can be listed as a candidate for such carbon. However, in such a field emission electron source having the cold cathode coated with the carbon nanotube, because of the large gas adsorption of the carbon nanotube, a problem exists that it is difficult to obtain a stable field emission due to significant and continuous degassing from the carbon nanotube under the vacuum operation.
In addition, there arises a problem in using the field emission electron source with coatings of the barium oxide or the carbon, where destruction of the field emission electron source occurs easily due to the induced discharge at the interface part of the coatings under a low electric field which is caused by a weakened interface.
Furthermore, there exists another problem in that it is difficult to obtain a large emission current J because the emission current is saturated at a low current level due to the occurrence of the contact resistance between the metal cold cathode and the thin film coating made of barium oxide or the carbon, etc.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. H 11-297189.    Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2000-208029.    Non-Patent Document 1: Tien T. TSONG, ATOM-PROBE FIELD MICROSCOPY and Cambridge University press, 1990, p. 110-115.    Non-Patent Document 2: F. Iwatsu and H. Morikawa and T. Tera, Journal de Physique (1987) Colloque C6-263, “An Attempt to Image Organic Molecules with FIM.”